As is known, the metering servo valve comprises a chamber for controlling of the usual rod for governing injection. The control chamber has a hole for inlet of the pressurized fuel, and at least one discharge hole, which is opened/closed by the open/close element under the control of an armature of an electromagnet. The discharge hole is opened when the armature is actuated by the electromagnet, overcoming the action of elastic means acting on the open/close element.
In known injectors, during closing of the servo valve, the open/close element is subjected to a train of rebounds of decreasing amplitude, against a detent that defines the position of closing of the discharge hole. In general, the first rebound is of considerable amplitude and causes a re-opening of the control chamber, with consequent temporary decrease in pressure, thus increasing the duration of the injection and hence the amount of fuel injected. Also the subsequent rebounds can further increase the volume of fuel injected.
Upon closing of the servo valve, globally the rebounds of the open/close element hence cause an increase in the amount of fuel injected with respect to the amount envisaged by the usual electronic control unit for regulating injection. In addition, the train of rebounds, which occurs in the presence of vapour, rapidly deteriorates the surfaces corresponding to the area of sealing of the servo valve, thus shortening the life of the injector. Finally, the mode in which this train of rebounds occurs depends upon many factors, amongst which the life of the servo valve. In fact, in the servo valves of the injectors there are fluid-tight dynamic couplings, characterized by surfaces that slide in relative motion with fits in the region of a few microns. Consequently, machining errors entail a certain friction in the first few hours of operation; then, on account of the inevitable wear, these surfaces present less friction and hence the amplitude and length of the train of rebounds is even more accentuated.
It will be understood in any case how all this jeopardizes the robustness of operation of the injector. In fact, on account of the large number in factors affecting the rebounds, the excess of fuel introduced is unforeseeable so that is not possible to compensate for it automatically, for example, by introducing a corrective factor for the time of energization of the electromagnet. Consequently, especially when the engine is idling, the excess of fuel causes a variation in the air-to-fuel ratio, which departs from the optimal one, causing at exhaust an excess of pollutant emissions into the environment.
Known from the document U.S. Pat. No. 5,820,101 is a fuel injector in which the spherical open/close element is controlled by an axial stem guided by a fixed bushing and is pushed by a first spring into a closing position of the servo valve. The armature is guided by said stem and normally rests against a detent carried by the stem on account of the action of a second spring. When the electromagnet is de-energized, the first spring brings the stem into a closing position, drawing the armature along with it. Upon arrest of the open/close element in the closing position, the armature continues its travel by inertia against the action of the second spring, which then brings it back into contact with the detent of the stem. Consequently, the armature is not able to reduce the rebound of the open/close element.
There also has been proposed an injector with metering servo valve of a balanced type, in which the open/close element in the closing position is subjected to axial actions of pressure that are substantially zero so that it is possible to reduce both the pre-loading of the spring and the force of the electromagnet. The valve body of this servo valve comprises an axial stem designed to guide axially the armature of the electromagnet, which is provided with a duct for discharge of the control chamber, which gives out onto the side surface of the stem. The open/close element is formed by a bushing made of non-magnetic material, which engages in a fluid-tight way with the stem. The armature is fixed with respect to the bushing, from which it is separate, and is made of magnetic material in order to simplify production thereof.
Instead, since the bushing must form a seal with the side surface of the stem, and since the open/close element must close the discharge duct via engagement with an annular detent, requires an extremely precise machining, on a very hard high-quality material.
In this servo valve, even though the stroke of the open/close element is of just a few microns, the forces and accelerations involved always entail at least one rebound of the open/close element during closing. The rebound is favoured by the high levels of hardness of the parts, by the presence of vapour associated to the flow of fuel in the presence of high pressure gradients, and by the reduced surfaces, which come into contact along a ring of a width of 1-2 hundredths of millimeter so that in general there occurs a re-opening and a corresponding emptying-out of the control chamber.
In addition, in known injectors the wear of the open/close element and of the corresponding arrest in the closing position of the servo valve, renders operation of the servo valve deterioratable during the life of the injector, since the closing travel of the open/close element and hence the duration of opening of the control chamber varies. Consequently, all the settings made in the control unit for governing the injectors are unable to take into account the variations due to wear, which are totally unforeseeable.